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An underbody is usually better than an air dam. Rather than moving through the air in as smooth a manner as possible, you are increasing frontal area (and possibly Cd) to try to influence how the air is moving, which regardless of the strength of the dam, and continuing under the vehicle.
I am not trying to discredit the airdam, just that an airdam is the way to go when an underbody can't be done. I couldn't believe anyone would choose (all things being equal) an airdam over an underbody.
On the topic of an airdam, everyone here makes good points. Strength of it is a pretty important factor when punching through the air, if it is deforming then there may be issues. I have no ABA testing, so take this with a grain of salt, but after my underbody was complete, at one point my dam was ripped off. There was no loss whatsoever in MPG, actually it increased. I can't provide data to a quantitative result, but from personal experience, an airdam or splitter is a negative when an underbody is clean.
(Although an underbody may in be place, air should still be diverted around the tires if possible. Jedi Sol has an OEM looking build for them, and others have done great jobs as well.)
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